At the present time, three main techniques are used for transmitting digital data over a network comprising a number of nodes, namely, message switching, packet switching and line switching.
Message switching consists in storing incoming messages received at each node of the network and in retransmitting them at some latter time over the proper outgoing lines. Packet switching is similar to message switching, except that incoming messages are first divided into portions, or "packets", which are then stored and retransmitted.
Line switching (either in space or in time) consists in setting up a physical circuit path between the incoming and outgoing lines. This technique is more widely used than either message switching or packet switching as it makes it possible, in particular, to handle very long messages, such as those generated by optical character readers or by digital voice coders, whereas the processing of such messages would require large size storage devices if the latter tecniques were employed. In practice, the use of message switching or packet switching is generally restricted to the more important nodes of a transmission network.
Line switching per se is well known. Any telephone call between two parties involves this technique. Line switching is also used for the transmission of digital data since many devices permit converting such data into signals that can be transmitted over telephone lines and retrieving the original data at the receiving end.
However, because a network frequently comprises long and/or numerous links, it may prove necessary, in order to maintain the integrity of the original data, to demodulate the signals received at certain nodes and to modulate them again before retransmission, resulting in a break of continuity at these nodes.
Regardless of the accuracy of the timing devices used in a transmission network, it is difficult to achieve the same data-signaling rate on both sides of each node. Various means have been proposed to synchronize all components of a network so as to obtain the same signaling rate on all links, but such means are costly. The present invention permits to achieve the same result without having to synchronize the components of the network. It should be noted that, in practice, the signaling rate from one link to another remains very close to the same value.
One way of facilitating the transmission of data is to use at each end of the network devices whose transmission speed is slightly lower than the lowest speed achieved on the various links. As is known, this makes it possible to add to the information elements, at the transmitting end, a sequence of digital elements the number of which may then be modified, that is, increased or decreased, at the different nodes so as to compensate for variations of the signaling rate.
Several known methods may be used to insert the additional elements in the information elements. For example, one may insert a given number of consecutive identical elements and take steps to ensure that the configuration of any part of the information elements will not be identical with that of the additional elements.
However, none of these methods allow a sequence of additional elements to be inserted in or removed from the information elements at any time.
In order that the transmission of the information elements may be transparent (that is, independent of their meaning) and that the additional elements may be inserted or removed at any time, thereby eliminating the need for using large size buffers, the following requirements must be met:
1. Insertion of the additional elements must be immediate, i.e., must take place as soon as an appropriate instruction is received.
2. Removal of the additional elements must be practically immediate, i.e., must take place within a very short time interval after receipt of an appropriate instruction.
3. The probability that the configuration of any part of the information elements will be identical with that of the additional elements must be low, and in the event of the two configurations being identical, it must be easy to modify the configuration of the information elements.